This invention relates generally to compressed gas transportation schemes and in particular to compressed gas delivery systems and methods for unloading compressed gas.
Natural gas is a colorless, odorless, fuel that burns cleaner than many other traditional fossil fuels. It is used for heating, cooling, production of electricity and it finds many uses in industry. Increasingly, natural gas is being used in combination with other fuels to improve their environmental performance and decrease pollution. It is one of the most popular forms of energy today.
The efficient and effective movement of natural gas from producing regions to consumption regions requires an extensive and elaborate transportation system. In many instances, natural gas produced from a particular well will have to travel a great distance to reach its point of use. The transportation system for natural gas consists of a complex network of pipelines, designed to quickly and efficiently transfer natural gas from its origin, to areas of high natural gas demand. Alternative transportation and delivery systems for natural gas, where pipelines are not possible or feasible, include the use of vehicles such as barges, ships, trains, and trucks. These systems generally consist of a multitude of high pressure storage containers on the vehicles and a delivery system which is connected to the storage containers at the delivery point.
Normally, delivery sites are designed for a distinct purpose and constructed to perform within specific parameters. For example, pipelines generally can receive higher pressure compressed gasxe2x80x94approximately 1000 psigxe2x80x94at variable flow rates from a particular delivery system, while power plants need a constant flow of natural gas at lower pressuresxe2x80x94approximately 300-400 psig. Location of the delivery site and materials used in its construction are also important variables that factor into designing an appropriate delivery system. For example, depending on the temperature and pressure tolerances involved, heat may be required during the delivery process.
In the latter transportation system using vehicles, once the compressed natural gas arrives at a delivery site (e.g. a power plant or pipeline), it is connected to a delivery system. Here, the high pressure compressed gas will initially flow on its own from the containers through the system towards a delivery point. This is because pressure at the delivery point is generally lower than the container pressure. In fact, due to the significant pressure differential between container pressure and delivery point pressure, some systems also provide a means of letting down the container pressure to slightly above pressure at the delivery point.
As the high pressure gas is delivered, the container pressure will eventually approach delivery point pressure. When the container pressure reaches the delivery point pressure, the residual gas left in the containers will no longer flow on its own. This residual container gas will remain in the containers unless it can be removed by other methods. For example, some systems use a compressor to compress the residual container gas up to delivery pressure, causing it to be removed from the system. These systems are inefficient because they require an independent source of energy to raise the pressure of gas from residual pressure to delivery pressure. This results in increased capital and operating costs.
Accordingly, there is a need for a system for transferring compressed gas from a supply point to a delivery point that optimizes the volume of gas delivered efficiently, regardless of the construction and operating parameters, while maintaining reasonable capital and operating costs. Other needs will become apparent in view of the following detailed description, in conjunction with the drawings.
It is to be understood that both the foregoing general description and the following detailed description are not limiting but are intended to provide further explanation of the invention claimed. The accompanying drawings, which are incorporated in and constitute part of this specification, are included to illustrate and provide a further understanding of the method and system of the present invention. Together with the description, the drawings serve to explain the principles of the invention.
The invention provides a system and a method for transferring compressed gas (e.g. natural gas, helium, carbon dioxide, hydrogen, etc.) from a supply point to a delivery point using an ejector, which works to maintain efficiency, without significantly increasing capital or operational costs. The system can be configured to transfer compressed gas from any number of containers or container banks. Accordingly, a multitude of system configurations of the present invention are possible. For example, one preferred embodiment of the present invention includes the following components: an ejector, a high pressure header, a low pressure header, flow and pressure control valves, flow and pressure indicating sensors, a control system, and a plurality of containers banks (or containers) holding compressed gas.
Initially, a required flow rate of compressed gas, corresponding to the fuel requirements of the delivery point (i.e. a power plant or the set point of flow into a pipeline) is entered into the control system. At the start of the delivery cycle, a first container bank is opened to the high pressure header. Compressed gas from the first container bank flows on its own to the delivery point, as it exceeds the delivery point pressure. Preferably, flow and pressure sensors continuously monitor compressed gas flow rates and system pressures and transmit this information to the control system. The control system processes this information, using it to operate flow and pressure control valves which throttle open and closed, thereby controlling the flow of gas to maintain system pressure and sustain the required flow rate.
Container gas flow will decrease as pressure drops in the first container bank, which will trigger the control system to open the next full container bank to the high pressure header and switch the first container bank from the high pressure header to the low pressure header. This configuration allows compressed gas from the high pressure header to be in flow communication with an ejector motive gas inlet, and allows compressed gas from the low pressure header to be in flow communication with an ejector suction inlet. As used herein, the term xe2x80x9cflow communicationxe2x80x9d means any method of connecting one structure to another which allows fluid to flow. The term flow communication includes not only direct connections, but also connections that may contain intermediate structures, such as valves, flow sensors, pressure sensors, splitters, etc. Accordingly, the amount of total gas flow out of the ejector will be a combination of the low pressure suction gas and the high pressure motive gas, all regulated by the control system.
As the pressure in the high pressure header drops, the amount of suction at the ejector suction inlet will drop, thereby requiring more gas from the high pressure header to maintain ejector output at the required delivery flow rate. Eventually, however, there will not be enough high pressure gas to effectively draw gas through the ejector from the low pressure header or deliver the compressed gas to the delivery point. At this point, the banks of cylinders will have to be switched again. This process will continue until all the container banks are at the residual pressure of the system.
Other embodiments of the present invention may include a compressor which can work with the ejector either in series or in parallel to ensure optimum ejector performance or constant delivery pressure. When the ejector is operating to maintain desired pressures and flow rates on its own, the compressor is turned off, thereby maintaining system efficiency and minimizing operational costs. For example, a compressor can be positioned in series, upstream of the ejector motive gas inlet or the ejector suction inlet, ensuring that compressed gas entering the ejector is constantly at optimum pressure. Alternatively, a compressor can be positioned in series downstream of the ejector or in parallel with the ejector, ensuring constant pressure at the delivery point. Unlike other systems, which do not use ejectors, the present invention uses a compressor only as necessary to assist in obtaining optimal ejector performance or a constant delivery pressure. It will therefore be appreciated that a compressor is not a necessary element of the present invention.